Processing of ceramic compositions or articles, e.g., sintering, annealing, or reaction bonding, by using one or more layers of low density, low-thermal conductivity insulation is known.
Providing silicon nitride articles, as well as other ceramic articles, e.g., electronic substrates of alumina or other similar materials, multi-layered capacitors, wear parts, dies or cutting tools formed from other ceramic type materials is of great interest. Silicon nitride articles are of particular interest and belong to a class of materials that have a wide range of compositions where Si.sub.3 N.sub.4 is the major phase and are especially of great interest in numerous applications due to their ability to withstand high temperatures and resist oxidation. These silicon nitride containing materials can be used in numerous applications for such diverse items as cutting tools, including inserts having various shapes and sizes, turbine and engine parts such as, rotors and stator vanes for advanced gas turbines, valves and cam roller followers for gasoline and diesel engines, and radomes on missiles to name a few. Some of these articles are of complex shape such as figurines, hubbed gears with teeth and object having variable cross-sections.
There are major differences between conventional thermal processing and microwave processing. To heat a part conventionally, i.e., thermal processing, only requires putting the part into a furnace and heating the furnace to the required temperature. If enough time is taken to permit heat to flow into the part, it must, by necessity, heat to the required temperature. Such is not the case when heating a material in a microwave furnace. Whereas heat is supplied externally to the part being processed conventionally, it is generated within the part being processed by microwaves. Depending on the dielectric properties of the part, several different phenomena may occur: (1) it may heat quickly; (2) it may not heat at all; (3) it may heat uniformly; or (4) it may generate hot spots. How the part heats depends on the dielectric loss properties of the part, how those properties change with temperature, on the microwave field distribution in the furnace and also on how the part is packaged for microwave heating.
In addition to chemical and thermal considerations, dielectric properties must be considered in microwave heating. This includes the material being processed, the insulation surrounding the part, and auxiliary materials such as thermocouples.
Uniform heating by microwave energy with the aid of a multi-layered low density insulation arrangement of the prior art has been unsuccessful because it leads to several problems. Development of an insulating system should give reproducible results and is the most difficult task in the high temperature processing of ceramics using microwaves. This is especially so when complex articles are processed.
Initially, as some articles heat-up in the presence of microwave energy, a reverse thermal gradient is generated from the inside of the part to the surface. This results in non-uniform densification and shrinkage, ultimately the article produced using low thermal conductivity insulation frequently cracks. Non-uniform heating of ceramic parts can also be a result of non-uniform microwave fields within the microwave cavity. The effect of non-uniform heating is most pronounced in articles having low thermal conductivity.
The most obvious example of these types of articles with low thermal conductivity are powdered compact samples with green densities of 45%-60%, where thermal conductivity is dictated by the porosity. Non-uniform heating of these types of samples can generate hot-spots and localize thermal run-aways. In addition, non-uniform densification, shrinkage cracking, and in some cases localized melting of the material occurs.
Previous attempts to overcome some of these problems involve the use of microwave succeptors to improve heating uniformity. These succeptors are for both internal heating within the parts or its external sources. For example, heating sources include particulates such as Al, C, SiC, TiN, TiC, Fe.sub.3 O.sub.4. External sources include succeptors such as SiC rods, commonly referred to as a "picket-fence". However, the prior art use of microwave succeptors has an undesirable affect by either reducing the microwave field, and thus reducing the benefits of microwave heating, or adding an unwanted additive to the material being fabricated.
The use of one or more layers of low density, low-thermal conductivity insulation to provide uniform heating has also been attempted. For example, U.S. Pat. Nos. 5,154,799, 5,013,694, and 4,810,846 to Holcombe exemplify microwave treatment of silicon based materials with a multi-component insulation system.
Holcombe, et al. '779 teaches a method of nitriding a refractory-nitride forming material. In Holcombe, a metalloid article is heated to a temperature sufficient to react the metal or metalloid with nitrogen by applying microwave energy within a microwave oven. The Holcombe '779 process surrounds the refractory material to be treated with a ceramic aggregate of granular material. For example, a silicon powder compact was prepared and placed in a boron nitride crucible containing silicon nitride with 2 weight percent yttrium powder. An aluminum fiber board was placed around the crucible. The resulting product was converted to approximately 78% silicon nitride. However, it has been determined that when the refractory compact was completely packed in silicon nitride powder, partial melting occurred during the exothermic nitridation stage. Also, prior art processes contain "hot spots" that contribute to sample cracking.
Holcombe '694 teaches microwave heating a ceramic composite that is enclosed by an yttrium thermal insulating package. In Holcombe '846, a microwave heating container 22 includes a bottom wall 24 that can be made of boron nitride. The top wall can be made from a similar material. The side walls are composed of graphite. A casket for containing the article to be treated is located within the container that includes the boron nitride bottom wall and top wall. The casket is formed of low-thermal conductivity alumina or silica. These processes, however, do not provide satisfactory sintered articles with adequate densification.
One of the objects of the subject invention is to provide a heat distributor system that overcomes the above heating problems in the prior art microwave processes. It is another object of the subject invention to provide a method of preparing sintered reaction bonded silicon nitride articles or ceramic articles that are uniformly heated by utilizing the subject improved heat distributor system.
As a result of using the multi-layer heat distributor system of the subject invention, the problems encountered in prior thermal and microwave heating processes have been overcome, "hot spots" have been minimized, sample cracking eliminated, and overall processing times have been reduced. Consequently, the yield of defect-free parts have been increased.
Improved heat distributors in microwave processing as described in the present invention, would also aid in the uniform heating of dense materials such as in the thermal processing of ceramics, glasses and polymers. Such heat distributors would minimize the thermal gradients that are normally present during microwave heating.